Method of restricting the burning of a solid propellant charge



Dec. 15, 1959 H sc r 2,917,424

METHOD OF RESTRICTING THE BURNING OF A SOLID PROPELLANT CHARGE OriginalFiled Oct. 28, 1947 TORS m v M v ATTORNEY5 United StatesPatent OOriginal application October 28, 1947, Serial No. 782,608. Divided andthis application June 17, 1950,

' Serial No. 168,742

4Claims. (Cl. 154-117) This invention relates to propellants of thesolid type used in rocket motors and the like; and more particularly itrelates to such propellants compounded from an alkyd resin base.

This is a division of my copending application Serial No.'782,608, nowPatent No. 2,780,996, filed October-28, 1947.

The principal object of the invention is to bond a liner to the walls ofsuch a propellant charge, so as to prevent burning between the liner andthe wall'and to restrict the burning to surfaces of the chargewhich areuncovered by the liner.

It has heretofore been a practice to form a solid propellant charge, inthe shape of'a stick, and then fit it within the chamber of the rocketor motor. This is commonly done, for example, by casting the propellantin a suitable mold. When such a propellant charge or stick is placedwithin the chamber'of the motor, burning would ordinarily occur not onlyon the-surface of the stick which is intended to burn, but also betweenthe sides of the stick and the wall of the'chamber. This gives rise touncontrolled burning-with indeterminately high pressures and danger tothe motor. To avoid such undesired unrestricted burning around "thesides of the propellant stick, a coating or liner has been applied bybonding to it a material which adheres to the propellant, therebypreventing access of flame or hot gases to the sides of the propellantstick. Propellant charges thus coated are commonly called restrictedcharges because theburning is thus restricted to surfaces which areuncoated. 7

It is-desirable in some rocket motor applications to use a propellantcharge having an alkyd resin base, for example, a charge compounded frominorganic'perchlorates suspended in alkyd resin matrices. In the case ofsuch charges having an alkyd resin base, difficulty has been experiencedin providing a liner which binds itself with sufiicient tenacity to thesurface of the cured thermosetting alkyd-vinyl resin.

Furthermore, the application of previously known liners to alkyd resinbase propellants has not usually permitted such alkyd resin basepropellant charges to experience the 'very wide temperature range ofwhich it is otherwise capable, without cracking. A great advantage ofthe use of an alkyd resin base charge is that it is capable ofwithstanding very low temperatures and very high temperatures withoutcracking and it is desired that the liner shall have the same desirabletemperature range without cracking.

In accordance with our invention we have discovered that a materialeminently suitable for a liner for alkyd resin base propellant chargesis one compounded from a mixture of thermoplastic resins; one of theresins in the mixture being a non-drying alkyd resin formed in generalby condensing a polyhydric alcohol, such as glycerol, glycol,polyglycol, polyglycerol, pentaerythritol, hexitol and propane 1,2-diol,with a polycarboxylic acid, such ICC - .2 as sebacic, adipic, maleic,phthalic, etc., which may, if desired, be modified'with a saturatedfatty acid, or acids of low degree of unsaturation, or a naturalnon-drying oil; and the other resin'in the mixture being a polyamidepolymerization product formed by ,thefcondensation-of dimerized ortrimerized linoleic or linolenic acid with ethylene diamine or otherliquid aliphatic diamines such as, for example, trimethylene diamine.Such resin mixtures as these are thermoplastic and can be melted andapplied to the propellant charge .by dipping the charge in the resinmixture while the resinis melted. The charge can be left in long enoughto allow a substantial layer to build up to a desired thickness aroundthe walls of the charge which are to be thus restricted, and then thecharge can be withdrawn from'the molten bath. When the material adheringto the surfaces has cooled, it solidifies and forms abasic'coatingclosely adherent to the charge. If desired, a second or third or otherdippings may be provided to obtain a thicker layer.

The outer surface of the liner thus formed may, if desired, be protectedby a suitable covering such as a glass tape or a sticky cloth'tape, asparticular conditions may indicate.

A liner so formed possesses'good tensile properties at very lowtemperatures such as 30 F.- and lower, and still possesses sufficientstrength and adherence to the charge even though the charge and liner besubjected to temperatures as high as 150 F. and up.

Owing to the close adherence and tenacity of the layer even underextreme heat, undesired burning irregularities are minimized orsubstantially prevented along the surfaces of the alkyd resin typecharge, in the presence of hot gases; and the burning will be restrictedto the desired burning surface which is left uncoated.

A composition for the restricting material that has been found eminentlysatisfactory for this purpose comprises a thermoplastic mixturecontaining from 70% to 80% by weight of polyamideresin formed bypolymerizing the reaction product of dimerized and trimerized linoleicor linolenic acid with ethylene diamine and from- 20% to 30% by weightof a non-drying alkyd resin, which is preferably modified as indicatedbelow. In a preferred composition the restricting material comprises amixture containing approximately by Weight of the polyamide resin andabout 25% by weight of the nondrying alkyd resin.

The preferred alkyd resin is a glycerol sebacate modi-, fied by a fattyacid such as stearic, palmitic and lauric acids, aliphatic acids of lowdegree of unsaturation such as maleic, fumaric, crotonic, oleic,isocrotonic, acrylic, angelic, tiglic, or with natural non-drying oilssuch as castor, coconut or cottonseed oils. Other forms of thermoplasticnon-drying, alkyd resins which. are suitable for this purpose are: Thecondensation product of phthalic anhydride maleic acid and glycerol; aresin com.- prising a modified condensation product of adipic acid and apolyhydric alcohol, preferably glycerol; and a condensation productformed bycondensing maleic an-' more solid, dense, liner. Such arestricting composition must be compatible with the resinous materialemployed in compounding the propellant and when applied must' adheretightly to thesurfaces of the cured'alkyd resin base propellant.

The restricting liner material should be heated before Patented Dec. 15,1959 it is ready for application to the propellant charge; and l thebest temperature to which it should be' heated lies between 360 F. and400 F., the optimum temperature range being between 385; F. and 395 F.,If the lining material is allowed to become h eated above 400 El the;resins in the mixture are apt to react with each other and formcompounds liners. I

A physical embodiment of the invention is shown in the attached drawingof which:

Fig. 1 shows in longitudinalcrpss section a propellant charge restrictedwith a lineraccording to this invention; and

Fig. 2 is an isometric viewshowing the liner of Fig. 1 with an addedcoating of tape to protect theouter surface of the liner.

The charge shown in Fig. 1 comprises a propellant charge 11, or grain asit isYco mmonly called, of a conventional cylindrical form. When thecharge is of the alkyd-vinyl resin and perchlorate type a suitablecomposition is asfollows':

which are not satisfactory for Percent by weight According to thisinvention the charge is protected by a thermoplastic layer 12 intimatelybonded to the surface of charge 11. Although only the single layer 12may often be sulficient, improvement is had by a' second layer, and sucha second layerfisshown as the thermoplastic substance 13,'m'ade toadhereto the surfaceof the liner 12, by a second dipping.

Fig. 2-shows the coated propellant charge of Fig. l protected fromabrasion by a confining wrapping of adhesive tape 14 such as clothtap'e,glass tape, etc.

A method' by which the liner may be appliedto the propellant grain isas=followsz The propellant grain may be supported on a convenientdipping meansisuch as a platform and lowered into a mixture of polyamideresin and modified alkyd resin whichhas been previouslyheated-t oa'temperature somewhere between 360 F. and 400 F. The cold propellantgrain-irnmers edfin the'hot resin mixture is allowed to remain in thebath until a layer of the desired thickness has been deposited upon thesurface. The grain is then removed from the bath and permitted to coolto ambient temperature. adhering to the first one may be' for med on'the propellant grain by redipping the grain which has already beencooled once, into the thermoplastic resin bath until the liner hasreached the desiredthickness. The grain is then removed from thebathand'permitted to cool.

An important attribute of the liner of this invention is that it adherestenaciouslyto the propellant surface, even when the propellant is of thealkyd-vinyl resin and perchlorate type; and it possessesa hard, flexiblesurface, and suflicient tensilestrength to prevent itfrorn abradingeasily. Another of its attributes is its. high melting point accompaniedby very low thermal conductivity.

The liner material, furthermore, is impervious to the effect of hotgasesenc'ounte'red within the reaction chamber. This permitsrestrictedburning to take place on the desired surface of thepropellant chargeduring the entire run. a

We claim: 7

1. The methodbf restricting the burning'o'f a'thermosetting alkyd-vinylresin and inorganic-perchlorate propellant charge comprising dipping thecharge ina bath consisting of a mixture of'thermoplastic resins,oneresin being the condensation product'of ethylene diamine with Thesecond liner closely an acid selected from the group consisting oflinoleic or linolenic acid, and glycerol sebacate modified with acompound selected from the group consisting of saturated fatty acids,acids of low degree of uusaturation, and non-drying vegetable oils,.said bath having been heated to a temperature of 360 F. to 400 F..permitting a layer "of the "thermoplastic mixture to depositon saidpropellant charge until a layer of sufficient thickness has been formed,andremovingthegrain from the bath and permitting the thermoplasticmixture to cool to ambient temperature. 7 r

2. The method of restricting the burning of a thermosetting alkyd-vinylresin and inorganic perchlorate propellant charge comprising dipping thecharge in a bath consisting of a mixture of thermoplastic resins, oneresin being the condensation product of ethylene diamine with an acidselected from the group consisting of linoleic or linolenic acid, andglycerol sebacate modified with a compound selected. from the groupconsisting of saturated fatty acids, acids of low degree ofuusaturation, and non-drying vegetable oils, said bath having beenheated to a temperature of 360 F. to 400 F., permitting a layer IO fthethermoplastic mixture to deposit on the propellant charge until alayer of sufiicient thickness has been formed, and removing the grainfrom the bath and permitting the thermoplastic mixture to cool, thenredipping the chargein said bath toform a second coating covering. thesurface of the liner and bonding itself thereto and removing the grainfrom the bath and permitting the liner to cool to ambient temperature.

3. The method of restricting the burning of athermosetting alkyd-vinylresin {and inorganic perchlorate propellant charge comprising dippingthe charge in a bath consistingofa mixture of thermoplastic resins, oneresin being the condensation product of ethylene diamine with an acidselected,fromdhegroup consisting of linoleic or linolenic acid, andglycerol sebacate modified with a compound selectedfrom thegroupconsisting of saturated fatty acids, acids of low degree ofunsaturation, and non-drying vegetable oils, said bath having beenheated to a temperatu'reoffiGOWF. to 400 F. permitting a layerof thethermoplastic mixture to deposit on the propellant charge until'a layerof sufiicient thicknesshas been formed, and removing the grain from thebath and permitting the thermoplastic "mixture to cool, then re-,dipping the charge insaid bath to form. a second. coating covering thesurface of the liner and bondingitself thereto, removing the grain fromthe bath and permitting the liner to cool to ambient temperature, andwrapping said liner with overlappinglayers of adhesive tape.

4. The.method :of-restrictingtheburning of a thermo-, setting alkydvinyl resin .and inorganic perchlorate propellant chargecomprisingidippingthe charge in a bath consisting of amixtureofthermoplasticresins, one resin being the condensationproduct of ethylenediamine with an acid selected from the group consisting of linoleic orlinolenic acid, andglycerolisebacate modified with a compoundselectedfrom the group consistingof saturated fatty acids, acids of lowdegree of uusaturation, and nondrying vegetable oils, said bath havingbeen heated to a temperature of -360 F. to 400 F., permitting a layer ofthe thermoplastic mixture to deposit on the propellant charge until -alayer of sufficient thickness has been formed, and removing the grainfrom the bath and permitting the thermoplastic mixture to cool, thenredipping the charge in said bath to form a second coating covering thesurface of "the liner and .bondingsitself :thereto, removing the "grainfrom the bath and'permitting the liner to cool to ambient temperature,and wrapping sai liner with overlapping layers of glass tape.

No references cited.

3. THE METHOD OF RESTRICTING THE BURNING OF A THERMOSETTING ALKYD-VINYLRESIN AND INORGANIC PERCHLORATE PROPELLENT CHARGE COMPRISING DIPPING THECHARGE IN A BATH CONSISTING OF A MIXTURE OF THERMOPLASTIC RESINS, ONERESIN BEING THE CONDENSATION PRODUCT OF ETHYLENE DIAMINE WITH AN ACIDSELECTED FROM THE GROUP CONSISTING OF LINOELEIC OR LINOLENIC ACID, ANDGLYCEROL SEBACATE MODIFIED WITH A COMPOUND SELECTED FROM THE GROUPCONSISTING OF SATURATED FATTY ACIDS, ACIDS OF LOW DEGREE OFUNSATURATION, AND NON-DRYING VEGETABLE OILS, SAID BATH HAVING BEENHEATED TO A TEMPERATURE OF 360*F. TO 400*F. PERMITTING